Drug Design_L5

Question 1

what is drug metabolism and features does it possess?

Answer 1

(1) Most metabolic products are less pharmacologically active, but different metabolites with different effects. Exceptions include:
metabolite is more active (e.g. prodrugs)
metabolite is toxic or carcinogenic, undesirable side effects are likely to be induced
(2) Drug metabolism involves pathways for the biosynthesis of endogenous substrates (eg hormones, cholesterol and bile acids); drugs resemble the natural compound. In most cases, drugs are detoxified.
(3) The liver is the major site of drug metabolism, but specific drugs may undergo biotransformation in other tissues.
(4) Drug metabolism is required to convert lipophilic compounds into more hydrophilic compounds to be excreted. If the lipid soluble non-polar compounds are not metabolised, they will remain in the blood and tissues and maintain their pharmacological effects for much longer.

Question 2

classify the drugs according to their hydrophobicity and how are they digested in the body?

Answer 2

(1) hydrophilic drug: absorbed in the liver and circulate in the bloodstream, eventually end up in the urine.
(2) lipophilic drug no metabolism: side effects will be incured if the drug stays in the blood circulation without being excreted.
(3) lipophilic drug: the drug is partially excreted out from the body and the remainder circulates in the blood until the secondary metabolism reactions convert them into the polar phase which is digestable.

Question 3

define the pharmacokinetics

Answer 3

(1) Absorbed, Metabolized,Distributed, and Eliminated (ADME)
(2) Orally administered drug dissolve in the GI tract and absorbed through the gut. Enter the liver and into the blood stream circulation. The end-product in the metabolic pathway is excreted from the body by urine or faeces.

Question 4

How does the pharmacokinetics influence the dosage?

Answer 4

(1) The physical and chemical properties of a drug determine its success to reach its target
1. Chemical stability
e. g. stable in the stomach?
2. Metabolic stability
e. g. no interaction with other metabolites
e. g. no reaction with the hormone in the body
e. g. clearance: how quickly the drug is metabolised
3. Toxicity
(2) Successful Absorption
e. g. the ability of crossing membranes
e. g. the concentration absorbed in order to be in the bloodstream
(3) two more questions need to be drawn from volume of distribution, clearance and the absorption: 1. half-life; 2. oral bioavailability. If the dosage is out of the range, either the toxicity overrides or the drug is cleared quickly by the enzyme.

Question 5

What are the phases of the drug metabolism?

Answer 5

(1) phase 0: the compound is absorbed into the intestine either by crossing the membrane directly or bring transported by the membrane protein OATPs
(2) phase I: the drug is metabolised by specific enzymes but is not ready for secretion
(3) phase II: further proceeding in the forms like glucuronides, sulfonates, GSH-conjugates
(4) phase III: excretion using the transporters

Question 6

describe the phase I and phase II transformation

Answer 6

(1) Phase I transformations – introduce or unmask a functional group, e.g., by oxygenation or hydrolysis (OH, -SH, -NH2, -COOH, etc.)
1. These metabolites are often inactive
2. Can be excreted readily though the urine
3. the compounds need firstly to be hydrophilic
(2) Phase II transformations – generate highly polar derivatives (conjugates) for excretion; glucoronide, sulfate, acetate, amino acids which all contain the sugar moiety but are too big to be secreted from the liver. If phase I does not convert the drug into soluble form, phase Ii is required.
(3) detoxifying mainly occurs in phase I and phase II

Question 7

introduce the oxidation in the phase I metabolism

Answer 7

(1) Addition of oxygen or removal of hydrogen.
(2) The first and most common step involved in the drug metabolism
(3) Liver is the main organ whereas oxidation takes place by cytochrome P450
(4) Increased polarity of the oxidised products; increased water solubility and excretion in urine
(5) the reactions include:
1. aliphatic or aromatic hydroxylation: e.g. benzene ring oxidation
2. N-, or S-oxidation
3. N-, O-, S-dealkylation: e.g. -S-CH3 ——> SH+CH2O
the introduced hydrogen dissolves the compound better

Question 8

introduce the reduction in the phase I metabolism

Answer 8

(1) Removal of oxygen or addition of hydrogen
(2) Less common than oxidation
(3) Cytochrome P450 system is involved in some reactions. Other reactions are catalysed by reductases present in different sites within the body.
(4) Nitro reduction to hydroxylamine/ amine(-NO2 to -NH2).
(5) Carbonyl reduction to alcohol(-COH to -CH2OH)

Question 9

introduce the hydrolysis in the phase I metabolism

Answer 9

(1) It is the reaction between a compound and water
(2) The addition of water gives more polar metabolites
(3) Different enzymes catalyze the hydrolysis of drugs:
e.g. esterase and amidase
(4) Ester or amide to acid and alcohol or amine
ester——> carboxylic acid+primary alcohol
amide——> carbocylic acid+amine
(5) water usually increases the hydrophilicity of the compound

Question 10

list enzymes for oxidation, reduction and hydrolysis

Answer 10

(1) Oxidation enzymes: primary enzymes in the liver
1. Cytochrome P450 monooxygenase system
2. Alcohol dehydrogenase
3. Aldehyde dehydrogenase
4. Flavin-containing monooxygenase system
5. Monoamine oxidase
(2) Reduction enzymes
1. NADPH-cytochrome P450 reductase
2. Reduced (ferrous) cytochrome P450
(3) Hydrolysis enzymes
1. Esterases and amidases
2. Epoxide hydrolase

Question 11

conclude for the phase I

Answer 11

(1) Microsomal mixed-function oxidase system (cytochrome P450 dependent); oxygen and a reducing system (NADPH) is required—one atom of oxygen is transferred to the substrate, and the other undergoes a two electron reduction and is converted to water
(2) The mixed-function oxidase is found in microsomes (endoplasmic reticulum) of many cells (liver, kidney, lung, and intestine), P450 mainly functions in the liver.

Question 12

describe Cytochrome P450 (CYP)

Answer 12

(1) Catalyses hydroxylation or epoxidation of various substrates
(2) Requires another enzyme NADPH-cytochrome P450 reductase, a flavoenzyme that contains one molecule of flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN).
(3) POR(P450 oxidoreductase) is the reaction centre for electron transfer to P450 for oxygen conversion to water.
(4) P450 contains a Heme which is coordinated with Cys435, or protoporphyrin IX, an iron(III) porphyrin cofactor
(5) Molecular oxygen binds to the heme cofactor after reduction of ferric (Fe3+) to ferrous (Fe2+: active form) and is converted to a reactive form which is used in a number of oxygenation reactions
(6) the oxygen is activated at the heme group for better attachment and modification, the incoming substrate is attacked.
(7) the large cavity of the P450 accomodates the substrate with low affinity, the substrates are trapped at the active site for modifcation by the enzyme as well as the other P450-associated enzymes

Question 13

What members are in the iron-oxo species?

Answer 13

proteins waste a vast amount of energy in distinguishing the species:

(1) Ferric heme
(2) Ferrous heme
(3) Ferrous oxy
(4) ferric peroxy,
(5) feric hydroperoxo
(6) ferryl oxo

Question 14

conclude for the phase II transformation

Answer 14

(1) When phase I products are not sufficiently hydrophilic or inactive to be eliminated, the drugs or metabolites formed from phase I reaction undergo phase II reactions
(2) Phase I reactions provide a functional group in the molecule to undergo phase II reactions. Phase II reactions modify functional groups by a conjugation reaction.
(3) Phase II conjugation reactions are capable of converting these metabolites to more polar and water soluble products.
(4) Require coenzyme
(5) Conjugation reactions are catalysed by transferases (liver)
(6) Most conjugates are biologically inactive and nontoxic because they are highly polar and unable to cross cell membrane

Question 15

What reactions can potentially happen in the phase II metabolism?

Answer 15

(1) Glucuronidation by UDP-Glucuronosyltransferase (-OH, -COOH, -NH2, -SH): attaching sugar moiety makes the secretion of the metabolites much easier
(2) Sulfation by Sulfotransferase: (-NH2, -SO2NH2, -OH): attachment of the sulphate
(3) Acetylation by acetyltransferase (-NH2, -SO2NH2, -OH)
(4) Amino acid conjugation (-COOH)
(5) Glutathione conjugation by Glutathione-S-transferase
(6) Fatty acid conjugation
(7) Condensation reactions

Question 16

describe glucuronidation reaction

Answer 16

(1) Conjugation to a-D-glucuronic acid
1. Most important phase II pathway for drugs and endogenous compounds
2. Products are often excreted in the bile
3. Enterohepatic recycling may occur due to gut glucuronidases
4. Requires enzyme UDP-glucuronosyltransferase (UGT)
(2) e.g. alpha-D-glucose 1-UDP conversion from the alpha-D-glucose 1- phosphate and incoming UTP with the release of PPi by phosphorylase, UDPG hydroxylase oxidises the alpha-D-glucose 1-UDP to alpha-D-glucose 1-UDP-glucoronic acid, meanwhile, 2 molecules of NAP+ are reduced to NAPH

Question 17

classification of glucuronidation

Answer 17

(1) N-glucuronidation
1. Occurs with amines (mainly aromatic ) which are introduced in the phase I, the product is more water soluble
2. Occurs with amides and sulfonamides
(2) O-glucuronidation
1. Occurs by ester linkages with carboxylic acids
2. Occurs by ether linkages with phenols and alcohols

Question 18

describe sulfation

Answer 18

(1) Major pathway for phenols (also alcohols, amines and thiols)
PAPS (3’-Phosphoadenosine-5’-phosphosulfate) which is the main enzyme required
(2) Sulfation at low substrate concentrations
(3) Glucuronidation at higher substrate concentrations, glucuronidation is prefered in the cell, but the chance of which reaction happens depends on the enzyme concentration

Question 19

define acetylation, fatty acid conjugation, amino acid conjugation and glutathione conjugation

Answer 19

(1) Acetylation:
1. Aromatic amines and sulfonamides
2. Requires N-acetyltransferase and the co-factor acetyl-CoA
3. Important in sulfonamide metabolism because acetyl-sulfonamides are less soluble than the parent compound and may cause renal toxicity due to precipitation in the kidney, acetylated compounds need to go through the faeces.
(2) Fatty Acid Conjugation:
Stearic and palmitic acids are conjugated to drug by esterification reaction. Bulky compounds can not be secreted from the kidney.
(3) Amino Acid Conjugation
1. Active CoA-amino acid conjugates that react with drugs by N-Acetylation: Glycine, Glutamine, Arginine
(4) Glutathione Conjugation
1. Tripeptide Gly-Cys-Glu; conjugated by glutathione-S-transferase (GST)
2. Conjugated compounds can subsequently be attacked by g-glutamyltranspeptidase and a peptidase
products can be further acetylated, the conjugated form behaves like peptide which can be cleaved by the protease.

Question 20

How do the chemical reactions modify the metabolic resistance?

Answer 20

(1) Making drugs more resistant to metabolism:
Remove functional groups susceptible to enzymes. If the drug is excreted too quickly, the dosage of the drug is not high enough to induce any changes.
(2) Making drugs less resistant to metabolism:
If a drug is too resistant to metabolism, it can pose problems as well (toxicity, long-lasting side effects). Add functional groups that are susceptible to metabolic enzymes, drugs may be hard to enter in the phase I metabolism

Question 21

examplify a membrane transporter for detoxifying

Answer 21

e. g. Efflux transporters detoxify cells from ‘toxic compounds’; eg P-glycoprotein
1. One of the primary proteins involved in multidrug resistance in the treatment of cancers
2. ATP-binding cassette (ABC) –superfamily- requires ATP hydrolysis to drive the export

Question 22

factors affecting the metabolism

Answer 22

(1) Rate and pathway of drug metabolism are affected by species, strain, sex, age, hormones, pregnancy, and liver diseases
(2) Drug metabolism is stereospecific
1. Enantiomers act as two different xenobiotics – different metabolites and pharmacokinetics
2. Sometimes the inactive enantiomer produces toxic metabolites or may inhibit metabolism of active isomer

Question 23

the binding sites for the drugs

Answer 23

(1) Drugs can bind to plasma proteins
(2) Only unbound compound is available for distribution into tissues
(3) Acidic drugs tend to bind to albumin which is found in the bloodstream. Basic drugs bind to alpha-1 acid glycoprotein.
(4) the frequency of taking the drug is related to the bioavailability

Question 24

Drug metabolism of Prodrugs

Answer 24

(1) Prodrugs are compounds that are inactive, but are converted in the body to an active drug by metabolic enzymes
(2) Improve membrane permeability
1. Esters. If a carboxylic acid is important for drug binding to its target, but it prevents the drug from crossing a membrane, temporarily “hide” it as an ester. Once in the blood, it is hydrolysed to the active form by esterases, more hydrophilic after passing through the phase I metabolism
2. N-methylation- N-demethylation is a common liver metabolic reaction, amines may be methylated to increase hydrophobicity. These N-methyl groups will be removed in the liver. For example, the levodopa is carried by the membrane transporter across the membrane, once crosses, the decarboxylase removes the carboxyl group from the molecule and produces the hydrophilic dopamine which is too polar to pass through the brain blood barrier. Once across the barrier, the prodrug is activated as it is converted into the biologically effective form
3. Membrane transporter- mimic substrate to cross membrane.
(3) Extend life- 6-mercaptopurine is an immune suppressant (organ transplants), but is eliminated from the body quickly. A prodrug that slowly is converted to the drug allows longer activity.
(4) Less toxic or side effects- Salicylic acid is a painkiller, but phenolic -OH causes gastric bleeding. Aspirin has an ester to mask this toxic group until it is hydrolysed.